1. Field of the Invention
This invention relates generally to infrared measurement techniques. In particularly, this invention relates to the measurement of distant infrared sources in the far infrared region of the atmospheric window.
2. Description of the Prior Art
A wide variety of instruments are available in measuring infrared radiation in the confines of the laboratory. A smaller number of field instruments have been build and are commercially available which employ an infrared detector mounted in an evacuated DEWAR bottle equipped with a suitable window and having provisions for mounting spectral filters thereon. Such detector outputs are calibrated against the radiation emitted by a black body of a known temperature. When such a detector is calibrated and taken to a field location in a warm earth environment difficulties develop with the calibration technique. That is, distant sources produce very small signals at the detector and, when a chopper is used to help distinguish signal from background, the resulting waveform is a sine wave of which a considerable amount of noise is superimposed. In such environments the laboratory technique of using phase-lock processing circuitry and long integrating times to allow the accurate data is inconvenient and impractical. For field applications, a more compact and less costly technique required is for substracting dc signals from the background-thermal noise-signal.